ACF Autosave & Live Preview (using sessionStorage)

admin.js

(function($) {
  $(document).on('change', '.acf-postbox [id^="acf-field"]', function(e) {
    e.preventDefault();

    var permalink = $('#sample-permalink a').attr('href');

    if (!sessionStorage.getItem(permalink)) {
      sessionStorage.setItem('permalink', permalink);
    } 
    
    $('input[type="submit"]#publish').val('Updating...').addClass('button-disabled');
    
    $.ajax({
      method: 'post',
      url: hm_autosave.ajaxurl,
      data: {
        action: 'hm_autosave',
        post_id: $('#post_ID').val(),
        acf_name: $(this).closest('.acf-field').data('name'),
        acf_value: $(this).val()
      }
    }).done(function(message) {
      $('input[type="submit"]#publish').val('Update').removeClass('button-disabled');

      var win = window.open(
        sessionStorage.getItem('permalink'), 
        'Preview Window', 
        'height=1680,width=1200');
      win.location.reload();
    });
  });
})(jQuery);

functions.php

<?php

function hm_register_admin_scripts()
{
  $theme_version = wp_get_theme()->get('Version');

  wp_enqueue_script(
    'admin-js',
    get_template_directory_uri() . '/admin.js',
    array(),
    $theme_version,
    true
  );

  wp_localize_script(
    'admin-js',
    'hm_autosave',
    array(
      'ajaxurl' => admin_url('admin-ajax.php')
    )
  );
}

add_action('admin_enqueue_scripts', 'hm_register_admin_scripts');

function hm_autosave()
{
  if ($_REQUEST['post_id'] && $_REQUEST['acf_name']) {
    update_post_meta(
      esc_attr($_REQUEST['post_id']),
      $_REQUEST['acf_name'],
      $_REQUEST['acf_value']
    );
  }
  exit;
}

add_action('wp_ajax_hm_autosave', 'hm_autosave');
add_action('wp_ajax_nopriv_hm_autosave', 'hm_autosave');